Photocontrolled Energy Storage in Azobispyrazoles with Exceptionally Large Light Penetration Depths

Azobispyrazole, 4pzMe-5pzH, derivatives with small terminal substituents (Me, Et, i-Pr, and n-Pr) are reported to undergo facile reversible photoswitching in condensed phases at room temperature, exhibiting unprecedentedly large effective light penetration depths (1400 mu m of UV at 365 nm and 1400 mu m of visible light at 530 nm). These small photoswitches exhibit crystal-toliquid phase transitions upon UV irradiation, which increases the overall energy storage density of this material beyond 300 J/g that is similar to the specific energy of commercial Na-ion batteries. The impact of heteroarene design, the presence of ortho methyl substituents, and the terminal functional groups is explored for both condensedphase switching and energy storage. The design principles elucidated in this work will help to develop a wide variety of molecular solar thermal energy storage materials that operate in condensed phases.